Optical fiber cables that are commonly used for telecommunication purposes are comprised of a large number of fiber strands, each strand providing an independent optical pathway. The individual strands of the cable are color-coated to facilitate identification. Thus, one strand is colored blue, another one red, and so on, in much the same way as insulated sheath of conventional copper conductors are color-marked for quick visual identification.
The color-coating process consists of depositing on the fiber strand a suitable ink that is subsequently cured under the effect of ultra-violet (UV) radiation. The ink coating and curing are performed as a continuous process. The strand is drawn at high speed, in the range of up to 1000 meters per minute through an ink bath designed to deposit a uniform layer of ink material on the surface of the strand. Immediately thereafter, the stand enters the curing chamber where it is subjected to a dose of UV radiation sufficient to cure the coating.
It is well-recognized in the art that the curing process should be performed in an atmosphere substantially free of oxygen to avoid oxidation problems of the ink and/or the glass substrate that may affect the durability of the fiber. Typically, the oxygen content in the curing chamber is controlled by injection of suitable inert gas, such as nitrogen. This technique, however, has not been particularly successful in reducing the oxygen content of the curing chamber to safe levels. This is due to the air pumping action that is generated by the fiber as it moves at high speed through the chamber. Indeed, as the fiber moves through the air, before entering the curing chamber a very thin layer of air adheres to its surface and moves at about the same linear speed as the strand. Thus, when the strand enters the curing chamber, an appreciable quantity of air is entrained with it and contaminates the curing zone.